<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			/*
			此时，b的大小正好覆盖父级元素
			*/
			.a{
				height: 200px;
				width: 200px;
				border: 1px solid;
				position: relative;
			}
			.b{
				position: absolute;
				left: 0;
				right: 0;
				top: 0;
				bottom: 0;
				background-color: #CCCCCC;
			}
		</style>
	</head>
	<body>
		<div class="a">
			<div class="b"></div>
		</div>
	</body>
</html>
